At one point in the film, Jake (Jack Nicholson) is talking with Evelyn (Faye Dunaway) about having been a cop in Chinatown earlier in his career.  When she asks "What did you do there?", he smiles (he knows he is telling a joke) and says "As little as possible".  He means that he winked at the corruption he saw there, and compromised his ethics by not enforcing the law (in this case the assumption in the film is that the laws he did not enforce were good laws, which should have been enforced).  So he compromised his integrity, something which he is not particularly troubled by at this point in the film.

But then he learns the truth about Evelyn and her father Noah Cross, and determines that this is one time in his time when he will not wink at corruption, but will try to do the ethical thing.  When it all goes terribly wrong despite his best intentions, he is left standing in the street, stunned and shattered.  At that point, one of his cop partners says to him in an attempt to console him: "Forget it, Jake, it's Chinatown", and the image is of the residents of Chinatown sweeping around them as they stand and engulfing them, while we listen to that great score.

Chinatown is a physical place, but more importantly also a "place" in which we either do or don't compromise ourselves and our ethics, and in which sadly corrruption can have overwhelming power.

A great script, and a great movie.

I don't, however, agree with it anymore; I have since I first saw it in my early twenties come to a fuller understanding of the tremendous power that ethical behavior has to win over corruption.  I know now that the good guys can win.